Query about locking rear end - Diesel Forum - TheDieselStop.com
'11-16 General Questions General questions related to 2011-Up Super Duty trucks. If it doesn't fit the other categories, post it here. Gas engine discussion that pertains to all models is allowed. Specific gas engine questions should use the Gas Engines forum.

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post #1 of 7 (permalink) Old 01-26-2019, 08:40 AM Thread Starter
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Query about locking rear end

Mine 2011 SD.

Question: What is function of locking rear end? When to use it? When not to use it? Turning while locked - ok to do so, how tight of turn allowed?
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post #2 of 7 (permalink) Old 01-26-2019, 09:25 AM
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Simply put:

Additional Traction

When you need additional traction engage it. If it's not engaged power is only going to one rear wheel.

You don't use, need or want it when you have good traction.

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post #3 of 7 (permalink) Old 01-26-2019, 10:38 AM
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Locking rear diff is use when doing donuts in the school parking lot. Use when your friends are around. Don't use when cops are around. Turn as tight as you want just don't hit the lights. https://en.wikipedia.org/wiki/Locking_differential
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post #4 of 7 (permalink) Old 01-26-2019, 11:25 AM
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As DKR said, use it when you need additional traction mostly when you're off road. The way it works on these trucks is you can turn it on and it will engage below 20 MPH, over that speed and it's automatically disengaged. If it's on when you are turning on dry pavement you will feel it chirping the rear tires because the rear axles are locked together. It would be a good feature for doing donuts but when it's on the traction control is on even if you have it switched off. I guess the guys at Ford don't want you having too much fun.

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post #5 of 7 (permalink) Old 01-26-2019, 11:37 AM
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Originally Posted by DKR View Post
If it's not engaged power is only going to one rear wheel.
That is not how an unlocked differential works. When unlocked power will go to both wheels unless one loses traction. Then all of the power goes to the wheel without traction.

As long as neither wheel is spinning both wheels are powered with an open differential.

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post #6 of 7 (permalink) Old 01-26-2019, 10:53 PM
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^^ What he said ^^

I'll add that an open differential applies equal torque to both wheels. If you jack one side up, or otherwise limit its ability to deliver torque to the ground the other wheel sees the same limited torque and you won't go anywhere. No power is really going to either side, it's just that one is spinning and doing nothing with it. Engage the locker and you're no longer limited to the torque the worst wheel can deliver. Both sides will turn the same rpm and the side with good traction can get you moving (if the other axle isn't buried too deep ).

Of course the traction control system makes the open diff act like a good limited slip. It will apply the brake on the side that's spinning. This creates torque on the spinning side. Since the side with good traction sees the same torque it can get you moving. Just don't romp too hard on the go pedal or the traction control system will take over the throttle.

As others have mentioned it's not a good idea to engage the locker when you really don't need it.

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post #7 of 7 (permalink) Old 01-27-2019, 04:19 PM Thread Starter
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Thanks guys. Good info.

However, long ago, my concern is once while retrieving our RV out of open lot storage, and after a rain, pulling it uphill on grass to paved drive, I would engaged the electric locking rear. While pulling RV uphill, had to make right turn once on the pavement. What I heard was kinda loud clicking or knocks from the rear, while moving forward slowly, in that turn.

I understand how Rear end differential work, being a mechanic myself. And understand limited slip rear types, with clutch. I did drag racing in my youth, learned to move forward a bit, then back straight and pull forward some. That puts differential transfer to both wheels yielding better traction.

But this electric lock has me baffled. Guessing can only use when pulling straight ahead, no turning, like stuck in mud or pulling boat trailer on wet ramp . Yet some of you mention can do burn outs in circles! One other person on this forum said some Time ago, “could snap axles”.

Maybe something wrong with this setup when put in lock mode? Needless to say that loud sound from the rear scared me, and resorted to not using it, except for straight ahead movements. Or not at all. Hoping someone could shed more light why I heard, felt that noise, action on that turn.

And oh, the truck suffers from that harmonic bouncing at 42-49 mph. Instead of “keep on trucking”, add “and bouncing” with it!
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